1. Field of the Invention
The present invention relates to a device for controlling an internal combustion engine with a variable valve timing system.
2. Description of the Related Art
In order to purify the exhaust gas immediately after the start of the engine, a catalytic converter arranged in the exhaust system of the engine must be activated quickly. For this purpose, in a control device disclosed in Japanese Unexamined Patent Publication No. 2000-170556, the exhaust valve is temporarily opened in the expansion stroke by using a variable valve timing system, and thus the exhaust gas at a high temperature, in the expansion stroke, is allowed to flow into the catalytic converter in an attempt to raise the temperature of the catalytic converter to an activating temperature quickly.
By the way, in general, combustion is unstable just after the start of the engine. To stabilize the combustion, therefore, the combustion air-fuel ratio is selected to be richer than the stoichiometric air-fuel ratio and, hence, the exhaust gas contains unburned fuel in a relatively large amount. According to the above control device, therefore, the catalytic converter can be activated quickly as compared to when it is normally heated. It is, however, advantageous to activate the catalytic converter quickly by burning the unburned fuel in the exhaust gas exhausted in the exhaust stroke in the catalytic converter by using the secondary air rather than by raising the temperature of the catalytic converter by utilizing the exhaust gas temperature only, though the temperature of the exhaust gas exhausted in the exhaust stroke may be lower than that exhausted in the expansion stroke.
However, a pipe for communicating the air cleaner with the engine exhaust system, and a pump for supplying the secondary air to the engine exhaust system, cannot be easily mounted on a vehicle.